Conventionally, as an image processing apparatus of this type, a so-called page printer is known. In this apparatus, image processing is performed while holding raster data for one page in a raster memory. The raster data in such image processing apparatus includes not only text but also various images such as a simple figure and a photograph.
In recent years, the resolution of this image processing apparatus has improved, and the memory capacity necessary in accordance with the improvement in resolution has increased. For example, in a resolution of 600 dpi, for generating data for an A4-sized page, a 4 MByte memory is required. The tonality, with which 1 pixel has been conventionally represented by 2 levels, has improved from 16 (4 bits) to 256 (8 bits) levels, thus requiring a larger raster memory. Further, recently, color data is handled in many cases. In such cases, in YMCK space, in comparison with a case of monochrome image processing, the memory capacity for further 4 planes is required, therefore, the memory size is further increasing.
To cut costs increased by such increase in memory size, various memory-saving techniques are proposed. For example, the quantity of a raster memory is suppressed by holding raster data of a band-size and encoding the raster data of bands.
However, in known decoding methods, it is necessary to temporarily store decoded data in a buffer before video transfer is performed to a printer engine. To prevent data underflow to the print engine, it is necessary to perform parallel processing to video transfer one band from one buffer to the printer engine while decode the next band in another buffer. According to the method, two work buffers are necessary, and in comparison with a decoding method in which a work buffer is not required, redundant memories are required. Thus the memory area for storing coded data is reduced.